1. Field of the Invention:
The present invention relates to an equipment for measuring the length or the refractive index of a dielectric element for the transmission of signals of optical frequencies.
2. Description of the Prior Art:
It is known that the availability having dielectric elements of very low attenuation in the optical wavelengths has made it possible to use of such elements in systems for the transmission of light over more or less long distances. In these systems, the use of these dielectric elements requires for them a protection or covering in the form of cables. For various reasons (cabling, utilization, etc.), one may have to know precisely certain parameters of a dielectric element, in particular its length and its refractive index.
The dielectric element to be measured is constituted by an optical fibre, that is to say an element composed of one or more dielectrics such as glass, silica, plastic material, etc., but in all cases capable of conducting light.
This optical fibre may form part of a conductor constituted by one or more fibres transmitting the same signal in parallel, or else of a cabling element constituted by one or more conductors or even of a complete cable constituted by one or more such cabling elements.
It is known in the art to use a telemetric device for measuring distance in the air, such device comprising a light impulse laser, the impulses of which are reflected by a target, and transmitted therefrom over a first path to a photomultiplier and an oscilloscope, these same impulses being also transmitted along a second path to a photoelectric cell and to said oscilloscope. In this arrangement, the transmission of the light impulses is effected in air, whereas in the invention, as explained hereinafter, the transmission of said impulses is effected in a refracting medium having a refractive index of value n. It is therefore obvious that such a known device does not permit the measurement of a refractive index.
It is likewise known to measure the load or the force applied to a test piece of a compressible medium by measuring the variation in length of said test piece due to said force, the measuring being effected with the aid of electric impulses applied by a crystal forming the base of the said test piece, said electric impulses being transformed into acoustic impulses in the said medium. The transmission of the acoustic waves in a medium always obeys a different law from those of light waves utilized in the invention.
To solve this problem, the present invention proposes a method characterized in that it consists essentially in making at least one measurement of the duration of passage (t.sub.1 -t.sub.0) of a light impulse over the length L of the element to be measured, having a refractive index n, and in applying the formula: EQU t.sub.1 -t.sub.0 =nL/c
where c is the velocity of light in vacuo, to obtain either one of the values of n or of L, knowing the other.